Prior attempts to achieve acceptable flame-retarded polyester/cotton blends have not met with commercial success. None of the treatments is practical from the consumer point of view, producing fabrics that have a very stiff hand. This is because in order to achieve the requisite flame-retardant properties, a high chemical add-on is required. This add-on makes the fabric stiff, masks the color of the underlying fabric, and often imparts an acrid or unacceptable odor to the fabric. In addition, the performance of the flame-retardant fabrics is often unreliable.
Most of the previous work conducted on flame-retardant polyester/cotton blends used a single chemical system that was targeted for the cotton component of the blend. The approach was to "load" the fabrics with a fire-retardant specific for cotton, for instance THPS [tetrakis-(hydroxymethyl)phosphonium sulfate]. It was not unusual with these earlier products to use from 30 to 35% of fixed chemical add-on in order for the polyester/cotton blend fabric to pass a bottom vertical flame test. Regrettably, however, the aesthetics of the finished fabrics were poor, as they had a very stiff hand and the appearance of a coated fabric. The add-ons used for these products were far in excess of the theoretically required amounts.
The application of THP/urea-precondensate/ammonia or THPOH/ammonia has been used commercially for a number of years. The process consists of applying the precondensate to cotton fabric and drying the fabric to about 10 wt. % moisture. The precondensate is insolubilized by the ammonia inside the cotton fibers. Fixation of the precondensate must take place inside of the cotton fiber to be durable through multiple launderings. However, attempts made in the past to use this process to finish polyester/cotton fabric have not been successful when the polyester fiber content was greater than about 10 weight percent.
While there is an upper limit for the amount of the flame-retardant chemical that can be packed in the cotton fiber, techniques can be used to maximize that amount. It is generally believed that approximately 3 wt % phosphorus in the form of the THP/urea-precondensate/ammonia complex can be fixed inside of the cotton fiber; however, the actual amount will depend on the prior history of the cotton fiber. Fixation of the FR polymer inside the cotton fibers provides no protection to the polyester fibers. Therefore, the polyester fibers still need an additional chemical treatment to provide adequate flame resistance to the polyester/cotton fabric.
It has now been found that the hand and durability of polyester/cotton blend fabrics can be further improved by applying the THP/urea-precondensate/ammonia treatment after the application of a flame retardant specific to the polyester component, which protects the polyester fibers, and prior to the application of THPS/urea which is also used to protect the cotton fiber.
When THPS is applied to a polyester/cotton blend, it is generally believed that about 3% of fixed phosphorus in the form of the THP/urea-precondensate/ammonia complex is required to achieve flame-resistance results. Since the THPS is specific to the cotton, it does not react with the polyester content of the fabric but simply physically coats the polyester. As a result, after multiple launderings, that portion of the flame-retardant surrounding the polyester fiber is partially lost. In consequence, it was not unusual to use as much as 5.5% phosphorus add-on for a polyester/cotton blend, at least initially, in order to result in the target 3% of fixed phosphorus after 50 launderings in hot water.
In the 1970's, polyester/cotton blends were flame retarded using tris-2,3-dibromopropyl phosphate ("Tris") in combination with THPS. However, "Tris" was found to be a carcinogen and was withdrawn from the market, so that there is no predominantly-polyester blend of polyester and cotton sold today that has been treated with flame-retardant chemicals.
The textile literature contains references generally describing the use of two specific flame retardants for a blend of fibers, one for each component of the blend. It is reported that various approaches to the treatment of polyester/cotton blend fabrics have not been commercially accepted.
The flame-resistant polyester/cotton fabric of the present invention exhibits an improved hand and increased durability over multiple launderings and as such represents an improvement over the process described in commonly-assigned application Ser. No. 870,892 filed June 5, 1986, the disclosure of which is hereby incorporated by reference.
The object of this invention is to produce a flame-resistant polyester/cotton blend fabric having improved durability and hand by means of a multiple step application process, an intermediate step to maximize the location of the FR chemicals inside the cotton fibers in order that enough total phosphorus for good flame resistance and durability can be added to the polyester/cotton blend while, at the same time, producing a FR polyester/cotton fabric which has a greatly improved hand over the prior art.
An object of the present invention is to produce acceptable flame-resistant polyester/cotton blends using multiple flame-retardant chemicals or chemical systems in a specific sequence, and to employ processing conditions or adjuvants that produce a commercially acceptable, attractive product having good color, acceptable hand and commercially acceptable durability through multiple launderings.